Notice when it happens
Record whether the smell is strongest at startup, remains while the system runs or returns after the unit has been off. This pattern gives a technician more useful information than a guess about the cause.
Look beyond the filter
The coil, fan barrel, drain pan and internal housing sit beyond the removable filter. Moisture and accumulated material in accessible areas can require a deeper clean than routine filter care.
Avoid masking the evidence
Strong fragrances can cover an odour temporarily without addressing the condition found. Do not spray household chemicals through the vents or into electrical areas.
The practical takeaway
When an odour returns after the filter is clean, inspect the wider system rather than repeatedly masking the smell.